Automated prepress workflow systems have changed the way prepress operations occur by performing a variety of prepress operations in a digitized file with a computer system. In the past, manually altering physical documents or document representations was the norm. Modern workflow systems are now implemented in software and can perform a greater variety of operations on a digital document prior to producing it with a printing device. For example, a prepress workflow system may scale, rotate or otherwise manipulate an image that is included in a document, format text into a particular font, size, color, language or orientation, or the like. However, as document preparation software applications have become more sophisticated, workflow software systems have focused increasingly on managing printing efficiencies so that the cost of printing a document may be known ahead of executing a print job. This also allows for the estimation of the cost of a prepress workflow and printing operation to be known so that a profitable business model may be realized. This is especially important for color print jobs that use relatively expensive color inks and toners. Such workflow efficiencies have, by and large, allowed for the successful advent of digital print shops. Example prepress workflow or workflow related systems include those described in U.S. Pat. No. 6,295,133 to Bloomquist et al; U.S. Pat. No. 6,378,983 to Ito et al; U.S. Pat. No. 6,380,951 to Petchenkine et al, U.S. Pat. No. 6,411,396 to Benson et al; and U.S. Pat. No. 6,483,524 to Petchenkine et al, each of which is hereby incorporated by reference.
Page description language (“PDL”) based documents use languages such as Printer Command Language (“PCL”), PostScript (“PS”), and Portable Document Format (“PDF”) to control how PDL documents are printed on a printing device. In order to produce a finished sheet, the printing device interprets the data contained in the PDL file, renders objects within the data into bitmaps, and after all objects for the sheet are rendered, prints the sheet. Typically, the printing device includes a raster image processor or “RIP” that renders objects within the PDL file into bitmaps which are transferred to a print engine within the printing device. The print engine then renders the image onto some form of output media.
Workflow systems take advantage of the above described printing process in a PDL file by allowing operations to be performed on the PDL document just prior to printing (i.e. “pre-press”), such as, soft-proofing (i.e., displaying an exemplary digital rendering of one or more pages of a PDL document on a computer screen, display or other image-based system), adjusting the Pill file for various types of print media, controlling the printing output device to which a PDL document will be output, controlling the time and priority of pending printing jobs, arranging various pages in a PDL file to achieve a proper sequence or position of each page relative to other pages (herein referred to as “imposition” or “imposition processing”), and correcting colors so that the printed colors closely match computer display colors for the document prior to the document being submitted to a printing device for printing.
However, faithful reproduction of colors has always been a focus and challenge to the print industry. As is known, each reproduction device has its own color space, or “gamut” within which it can reproduce colors. When an image moves from one device to another, image colors may change because each device interprets color values according to its own color space. For example, it is impossible for all the colors viewed on a monitor to be identically matched in a printed output from a desktop printer. A printer operates in a cyan, magenta, yellow, and key (i.e. black) (hereinafter “CMYK”) color space, and a monitor operates in a red, green, blue (hereinafter “RGB”) color space. Their “gamuts” are different. Some colors produced by inks cannot be displayed on a monitor, and some colors that can be displayed on a monitor cannot be reproduced using inks on paper. Hence, the physical reality of color reproduction between computer systems, between ink based printing devices, and between color laser devices is greatly complicated. Nevertheless, workflow systems are designed to attempt to compensate for such physical differences.
Within any color PDL document, some colored sections in the document may use a color selected by the document creator called a “spot color.” A spot color is any color generated by an ink that is printed using a single pass or run through a printing device. The color can be any color, even non-standard offset colors such as metallic, fluorescent, or preprocessed colors. There are various methods to incorporate sophisticated patterns of spot colors in a final prepress work, and many software applications such as Adobe InDesign, Adobe Illustrator, and QuarkXPress can generate spot colors as additional “channels” in a PDL document to attempt to maintain consistency of printed document output. Some applications can also utilize spot colors in special layered printing effects such as feathered edges or dissolve effects.
For print devices that do not use special inks, the only option to reproduce a PDL document that calls for a spot color is to attempt to reproduce that color using a “process color.” In other words, the printer attempts to reproduce the spot color with a combination of colors in accordance with processing instructions built into the printer. However, the matching of color output of a spot color in a PDL document with printing devices that cannot provide the special ink of a spot color is complicated and troublesome, even beyond the technical differences in gamut between a computer monitor and a familiar printing device. And, quite often, the finished output of a document printed on a printing device will vary depending on a number of shifting variables, such as, for example, environmental influences like temperature, humidity, and elevation, variations in output media, and differences in toner or ink sources—all potentially leading to unsatisfactory print results. Added to these factors is the complexity that in most cases digital documents such as a PDF digital document print with different colors than what its creator viewed on a computer monitor. As mentioned above, this variation in color is due to the inherently different technologies used in representing color on computer monitors versus a printer. Because of the differences in how colors are shown, and because of the above described printing variances, no direct mathematical correlation exists between colors as presented on a computer monitor and colors printed through a print device.
Therefore, what is needed is a pre-process system for correcting spot colors in a PDL document so that the color output of a computer display and the color output of a printed device would be better matched, and so that the printed result would yield a consistently satisfactory color document.